Today GPS systems are relatively cheap and widely available. You don't even need a dedicated GPS receiver. For example, I often use a Android tablet with a built-in GPS, and software that can record waypoints and tracks on pre-loaded maps.
So the answer is, record the lat/lon coordinates of the spot you want to come back to. It can also help to record a GPS track on your way in and out. You can follow these tracks or get to the spot later without any alterations of the land.
Unless you own the land or otherwise have explicit permission,
- Leave flags.
- Make cairns.
- Cut blazes.
- Make chalk marks.
- Do anything else that mars the land, rocks, trees or anything else, or leaves a deliberate visible trace of you having been there.
All these are vandalism and just plain rude without permission of whoever has authority over the land. And no, it being public land doesn't give you the automatic right to do these things, at least not here in the US.
Also consider that maybe you shouldn't even be off a existing trail in the first place. Wandering around freely is allowed in many public lands, but staying on established trails is required in others. Don't assume you can wander off trail without making sure it is actually allowed on that particular property.
More on chalk marks
Some apparently believe that it's OK to go one someone else's land and mark it up with chalk, and even worse, indicate a whole new trail. In most case, that is not so. I would be upset if someone did that on my land.
Even in cases where the public has permission to walk on the land off-trail freely, marking a new trail still isn't right without explicit permission.
In my town, there are no general restrictions on walking around on town-owned conservation land. I am on both the Trails Committee (who does the planning, construction, and maintenance of trails) and the Conservation Commission (who is the authority that would grant permission for a new trail on such lands). I can tell you we'd take a dim view of anyone marking a new trail with chalk.
While the chalk marks alone are vandalism, the real issue is the new trail being marked. Others will likely see it and follow it, and now it becomes marked just from the wear of usage. It also gives others the impression that it's OK for anyone to mark a trail. This is not a sustainable situation, and would make work for the volunteers that manage the conservation lands and the trail system.
Most people not on the Trails Committee or the ConCom have no idea about what restrictions there might have been imposed on a property when it was acquired, deals made about trails with other entities holding CRs (conservation restrictions), research and thought that went into NOT putting a trail in a particular place, future plans for where trails will be, where particularly sensitive environmental areas are, etc. Someone just barging in can cause real damage. It can also derail negotiations between different groups about trail placements and the like.
What I mention above are not just academic points. These things have actually happened. In one case someone put a bridge across a ditch to make it easy to cross. We were in the middle of trying to convince the CR holder to allow us to put a bridge there. As you can imagine, a bridge suddenly appearing there without permission didn't go over well. Now all the trails in a whole section of that parcel are not allowed to be maintained and are essentially closed.
In another case, a trail was proposed and had gotten as far as being flagged with yellow tape, all with proper permission. It was then found this went too close to some vernal pools where the endangered blue spotted salamander was found. It was decided to not route a trail thru that area, and all the flags were carefully removed. We don't want anyone even giving the impression of a trail near those vernal pools.
Again, you simply don't have the right to mark a new trail on someone else's land without permission. Here in the US, you must assume that permission is not given unless done so explicitly.
Making a few chalk marks may seem harmless because they will be washed away after a while with no real environmental harm, but again, that's not the point. How would you feel if I put a bunch of chalk marks on your house? Sure, they'll wash away after a while, but that doesn't make it right.